1. Field of the Invention
The present invention relates to a multi-stage combined optical device employed between optical fibers, for transmitting light traveling in one direction, blocking light traveling in the opposite direction, and separating incident light power. More particularly, the present invention relates to a multi-stage combined optical device having an improved isolator-tab-photodiode (ITP) which can function as a plurality of optical isolators and can detect incident light power using a single optical isolator.
2. Description of the Related Art
Conventionally, a multi-stage combined optical device includes an optical isolator, first and second collimators disposed on light receiving and emitting planes of the optical isolator. The optical isolator is used for blocking a reverse ray using polarization modes. The collimators are used for focusing light incident on and output from first and second optical fibers, respectively.
A conventional optical isolator using polarization modes, positioned in the path of a light ray travelling between two optical fibers, transmits forward rays and blocks reverse rays. To achieve this, the optical isolator is comprised of a first polarizer made of a birefringent crystal with a wedge shape, a Faraday rotator, and a second polarizer. Light traveling in a forward direction travels in sequence from the first polarizer to the Faraday rotator and to the second polarizer. This forward ray is separated into an ordinary ray and an extraordinary ray while passing through the first and second polarizer and then directed to the optical fibers.
The difference between the path of the light corresponding to the ordinary ray and that corresponding to the extraordinary ray is defined as a walk-off. If the walk-off is large, a time delay between the ordinary ray and the extraordinary ray is produced. Such a time delay causes a polarization mode dispersion.
To solve the above-described problem, the applicant of the present invention has proposed an optical isolator using polarization modes, which can reduce walk-off without additional optical elements, in U.S. patent application Ser. No. 08/851,997 (hereinafter '997) which is incorporated herein by reference. The optical isolator disclosed in '997 suppresses the reverse ray by changing the paths traveled by the forward ray and reverse ray. Thus, the optical isolator is used in transmitting a channel signal, i.e., a single light signal. This optical isolator can be used in an optical amplifier such as an EDFA (Erbium Doped Fiber Amplifier). In such an optical amplifier an optical isolator for guiding an incident light, containing information, that travels in a forward direction must be provided. An optical isolator for suppressing a backwardly incident light must be provided separately, creating an impediment in making a compact optical amplifier.
Also, in order to detect the light power of an incident ray, a tap for transmitting most of incident rays and reflecting parts thereof and a photodetector for receiving light reflected from the tap are installed in a front end of the optical isolator.